1. Field of the Invention
The present invention relates to an AGC (Automatic Gain Control) circuit of an FM front-end portion and, more particularly, to an AGC circuit of an FM front-end portion used in an FM tuner of mobile audio equipment.
2. Description of the Related Art
An AGC circuit of an FM front-end portion is generally operated by receiving an input signal having an amplitude exceeding an AGC sensitivity, and an AGC amount is determined in accordance with a difference between the AGC sensitivity and the input amplitude.
The frequency characteristics of the AGC amount are shown in FIG. 1. For this reason, as shown in disturbance performance characteristics (when the AGC amount has the frequency characteristics in FIG. 1) in FIG. 2, when the AGC sensitivity is set to obtain improved disturbance performance in a close-detuning range, the AGC amount is very small (a solid line A in FIG. 1) in a remote-detuning range. As a result, the disturbance performance in the remote-detuning range is degraded (a solid line A in FIG. 2). In addition, when the AGC sensitivity is set to obtain improved disturbance performance in the remote-detuning range, since the AGC amount is too large in the close-detuning range, the disturbance performance in the close-detuning range is degraded (a solid line B in FIG. 2).
As described above, according to the conventional technique, since a trade-off relationship between the disturbance performance in the close-detuning range and the disturbance performance in the remote-detuning range is established, when one disturbance performance is improved, the other is degraded.